The present invention relates to a steel cord used for rubber product reinforcement, method of manufacturing the same, and a tire using the steel cord, and particularly to a steel cord effective for reinforcing a super-large off-road tire and an off-road tire using this.
In large-scale construction work such as dam construction, housing/road development and mining development, various operations such as excavating, cutting and leveling ground and carrying produced earth, sand and rock are indispensable. As means for carrying out these operations, motor graders, earth graders, earth movers, scrapers, shovel loaders, mobile hammers, power shovels, tire milers, load sweepers, dumper cars and mobile cranes and the like are used.
The vehicle constructions of these kinds (including cargo handling construction vehicles) are made larger and heavier in order to promote increased efficiency of the operations such as excavating and carrying for which they are used. In this connection the tires which support the loads of these machines also become larger, and super-large tires With a diameter over 4 m and the weight over 5 tons have come into use. These super-large off-mad tires, unlike tires for vehicles such as trucks which travel on road surfaces, travel on off-road surfaces where rocks and the like are exposed while supporting large loads exceeding 50 tons. Furthermore, the operation of replacing these tires is not easy. Consequently, it is required of these tires that they have good durability and as long a life as possible. To achieve this, the steel cord embedded in the rubber as a rubber reinforcing material must be a suitable one. In particular, because belt layers near the tread, and especially the outermost belt layer, are near the surfaces making contact with the ground, various stress fluctuations suffered during travel are large and they are subjected to impact type stresses when passing over projections such as rocks and the like. Because as a result the steel cords readily break, this has a large affect on the tire life. Therefore, steel cords used for reinforcing super-large off-road tires, and particularly those of belt layers near the tread, in addition to having good strength preferably have characteristics of pliability and large elongation (high ductility characteristic of a large elongation on breakage by tension) so that they can absorb impact type stresses.
For steel cords for belts of conventional super-large off-road tires, generally a (7.times.7)+1 structure wherein seven strands each made by twisting together seven steel wires are twisted together and one steel wire is wound around this in a helix has been used. However, with this kind of steel cord structure there has been the problem that the life of the steel cord is short because its elongation is low and it breaks easily.
As a countermeasure to this, the application of steel cord structures tried in steel cords of tires for trucks and buses, that is, 4.times.2 structures and 4.times.4 structures made by twisting together four stands ST each made by twisting together two or four steel wires SW, as shown in FIG. 1-A and FIG. 1-B, to super-large off-road tires has been thought of and in some cases actually used. However, in these steel cords, to obtain high ductility the lay lengths of the strands and the steel cord are made extremely short. For example, in a steel cord of 4.times.4 structure made up of steel wires whose diameter is 0.22 mm, the cord lay length has been short at about five times the cord outer diameter and the strand lay length has been short at about 1/2 the cord lay length. In this steel cord, the number of twists per unit length of cord is high and consequently the manufacturing cost is very high because the twisting efficiency is poor. Also there has been the shortcoming that because a decrease in strength caused by twisting loss is also large (the strength of the steel wires is not used efficiently) it is difficult to obtain a steel cord of high strength. Furthermore, because the breaking load of this type of cord currently in use is less than 1568N, it is necessary to make the number of cords embedded in reinforcing layers large and consequently there is the shortcoming that the tire weight increases.
For these reasons it has been possible to carry out satisfactory improvement by applying steel cords of tires for trucks and buses and steel cords of super-large off-road tires.